Descartes thinks he accumulated a lot of questionable and dubious ideas through his life. He makes a conclusion that all those ideas come from senses which are not trustworthy. It is easy to doubt credibility of our senses when they work on the edge of their sensitivity. For example, recognition of small or far away objects. It is much harder to doubt bigger chunk of our senses especially if they work in their confidence interval because their correctness could be proved by experience. If we are going to insist that some of our senses are wrong, we risk being considered as mad. However, it is much easier to reject our senses as a whole. For example, in a dream we perceive all weird events of the dream as normal and real, but we can recognize the strangeness of the dream only outside of the dream “reality” when we wake up. That is why Descartes decides to question the reliability of the whole world of senses.
If we accept an idea that our real world, given to us through senses, is just another dream of a higher rank, we may want to find criteria which would allow us to find imperfections of the dreams (especially of the lower rank) comparing to the really real world. Importance of these criteria is stressed, for example, in the motion picture Inception. For a moment Descartes follows this path, suggesting that the human fantasy is impaired by its scantiness: fantastical creatures that the humans make up are just a combination of parts of real animals, or images in dreams are like bleak paintings of real things. Descartes implies that if we dream a dream impressed on us by some “Architect” (in terms of the Inception), we can use “simple and universal” invariants of the really real world (like mathematical concepts) as landmarks for detecting a dream. But Descartes quickly withdraws from this path, suggesting that the “Architect” may be an omnipotent God capable of creating a deceptive dream for us, which is as perfect as the really real world (i.e. “totem” from Inception would not work).
Descartes meets possible counter-arguments that God could be non-omnipotent or could not be possibly deceptive because deception is manifestation of imperfectness, by saying that he has no answers to these objections. Descartes started doubting senses for he wanted to leave only certain and distinct ideas in his understanding of the world, therefore he is willing to build it suitable for the worst case scenario that God’s task is to deceive Descartes with all his might. If Descartes is still able to infer anything about the world even in conditions of omnipotent deception, those inferences would be quite certain and unshakable.
Descartes is able to identify at least one thing which omnipotent evil deceiver cannot possibly trick him about. He cannot make Descartes believe that he does not exist, therefore the statement “I am, I exist” cannot be taken away from him. Descartes finds other certain qualities of his “I”, which is a “thinking thing”, and which exists only while he thinks. His “I” also has senses, which does not mean that these senses are somehow real, and, strictly speaking, sensing is a type of thinking. “I” can have mental images, which are modifications of thought as well.
Another classification of thought is aimed at identifying what type of thought is prone to errors. Descartes divides thoughts into three categories. Two of them, which are simple ideas (even unreal and imaginary) and emotions, cannot be judged…